net/base/dns_reloader.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "net/base/dns_reloader.h"
   6
   7 #if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD) && \
   8     !defined(OS_ANDROID)
   9
  10 #include <resolv.h>
  11
  12 #include "base/basictypes.h"
  13 #include "base/lazy_instance.h"
  14 #include "base/logging.h"
  15 #include "base/message_loop/message_loop.h"
  16 #include "base/synchronization/lock.h"
  17 #include "base/threading/thread_local_storage.h"
  18 #include "net/base/network_change_notifier.h"
  19
  20 namespace net {
  21
  22 namespace {
  23
  24 // On Linux/BSD, changes to /etc/resolv.conf can go unnoticed thus resulting
  25 // in DNS queries failing either because nameservers are unknown on startup
  26 // or because nameserver info has changed as a result of e.g. connecting to
  27 // a new network. Some distributions patch glibc to stat /etc/resolv.conf
  28 // to try to automatically detect such changes but these patches are not
  29 // universal and even patched systems such as Jaunty appear to need calls
  30 // to res_ninit to reload the nameserver information in different threads.
  31 //
  32 // To fix this, on systems with FilePathWatcher support, we use
  33 // NetworkChangeNotifier::DNSObserver to monitor /etc/resolv.conf to
  34 // enable us to respond to DNS changes and reload the resolver state.
  35 //
  36 // OpenBSD does not have thread-safe res_ninit/res_nclose so we can't do
  37 // the same trick there and most *BSD's don't yet have support for
  38 // FilePathWatcher (but perhaps the new kqueue mac code just needs to be
  39 // ported to *BSD to support that).
  40 //
  41 // Android does not have /etc/resolv.conf. The system takes care of nameserver
  42 // changes, so none of this is needed.
  43
  44 class DnsReloader : public NetworkChangeNotifier::DNSObserver {
  45  public:
  46   struct ReloadState {
  47     int resolver_generation;
  48   };
  49
  50   // NetworkChangeNotifier::DNSObserver:
  51   void OnDNSChanged() override {
  52     DCHECK(base::MessageLoopForIO::IsCurrent());
  53     base::AutoLock l(lock_);
  54     resolver_generation_++;
  55   }
  56
  57   void MaybeReload() {
  58     ReloadState* reload_state = static_cast<ReloadState*>(tls_index_.Get());
  59     base::AutoLock l(lock_);
  60
  61     if (!reload_state) {
  62       reload_state = new ReloadState();
  63       reload_state->resolver_generation = resolver_generation_;
  64       res_ninit(&_res);
  65       tls_index_.Set(reload_state);
  66     } else if (reload_state->resolver_generation != resolver_generation_) {
  67       reload_state->resolver_generation = resolver_generation_;
  68       // It is safe to call res_nclose here since we know res_ninit will have
  69       // been called above.
  70       res_nclose(&_res);
  71       res_ninit(&_res);
  72     }
  73   }
  74
  75   // Free the allocated state.
  76   static void SlotReturnFunction(void* data) {
  77     ReloadState* reload_state = static_cast<ReloadState*>(data);
  78     if (reload_state)
  79       res_nclose(&_res);
  80     delete reload_state;
  81   }
  82
  83  private:
  84   DnsReloader() : resolver_generation_(0) {
  85     tls_index_.Initialize(SlotReturnFunction);
  86     NetworkChangeNotifier::AddDNSObserver(this);
  87   }
  88
  89   ~DnsReloader() override {
  90     NOTREACHED();  // LeakyLazyInstance is not destructed.
  91   }
  92
  93   base::Lock lock_;  // Protects resolver_generation_.
  94   int resolver_generation_;
  95   friend struct base::DefaultLazyInstanceTraits<DnsReloader>;
  96
  97   // We use thread local storage to identify which ReloadState to interact with.
  98   static base::ThreadLocalStorage::StaticSlot tls_index_;
  99
 100   DISALLOW_COPY_AND_ASSIGN(DnsReloader);
 101 };
 102
 103 // A TLS slot to the ReloadState for the current thread.
 104 // static
 105 base::ThreadLocalStorage::StaticSlot DnsReloader::tls_index_ = TLS_INITIALIZER;
 106
 107 base::LazyInstance<DnsReloader>::Leaky
 108     g_dns_reloader = LAZY_INSTANCE_INITIALIZER;
 109
 110 }  // namespace
 111
 112 void EnsureDnsReloaderInit() {
 113   g_dns_reloader.Pointer();
 114 }
 115
 116 void DnsReloaderMaybeReload() {
 117   // This routine can be called by any of the DNS worker threads.
 118   DnsReloader* dns_reloader = g_dns_reloader.Pointer();
 119   dns_reloader->MaybeReload();
 120 }
 121
 122 }  // namespace net
 123
 124 #endif  // defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_OPENBSD) &&
 125         // !defined(OS_ANDROID)